The present invention relates to improvements in cooling towers for condensing water or the like. More particularly, the invention relates to improvements in natural-draft or mechanical-draft cooling towers of the type wherein the central vertical section of the main wall of the tower is preferably a hyperbola so that the upper region of the main wall acts not unlike a diffusor and promotes the flow of ascending air.
Cooling towers often reach a height of up to and in excess of 100 meters and, in some instances, a height of up to and in excess of 150 meters. The diameter may reach or exceed 50 meters, in some instances 120 meters. Their main wall is often made of concrete or analogous construction material and is disposed above a water-distributing system which sprinkles water into a basin whereby the streamlets of water are contacted by air which enters the lower portion of the main wall and flows upwardly. It is also known to construct such cooling towers for cross-flow of gaseous heat removing medium. The towers may be designed for natural draft or mechanical draft; in the latter instance, the fan or fans can be mounted at the top of or below the main wall, i.e., the towers can be operated with forced draft or induced draft. A hyperbolical main wall exhibits a number of important advantages in that it enhances the stability and contributes to lower cost of the cooling tower. However, in presently known cooling towers with hyperbolical main walls, the aerodynamic factor has been neglected so that such towers are not entirely satisfactory, especially in the region of the upper portion of the main wall.
The manner in which air flows in the region of the upper portion of the main wall exerts a pronounced influence on the mode of operation and efficiency of the tower. The nature of air flow at the upper portion depends largely on atmospheric conditions, especially on the velocity of wind at the crown of the tower. Thus, in the absence of any wind or when the velocity of wind is relatively low but the wind blows in several directions, thermal instability develops at the upper end of the main wall which brings about a rather pronounced downward flow of cool air in the region between the ascending column of cooling air which entrains water vapors into the atmosphere and the internal surface of the main wall. The inflowing cool air reduces the cross-sectional area (i.e., it constricts the ascending column of vapor-laden air so that the efficiency of the cooling tower decreases well below the maximum or average efficiency. This will be readily appreciated since such constriction of the ascending column of vapor-laden air increases its speed so that the length of intervals during which the ascending body of air is maintained in contact with water in the cooling tower is reduced well below the optimum length. The acceleration of ascending air takes place in the entire region where the relatively cool atmospheric air is allowed to penetrate into and to flow downwardly along the internal surface of the main wall. This means that once-cooled water must be recycled through the tower because the maximum temperature of water which can be reused in a plant or which can be released into rivers or ponds is prescribed in most industrial countries and is vigorously enforced by authorities. The main reason for the just discussed thermal instability in the region of the upper end of the main wall in a cooling tower wherein the main wall has a hyperbolical vertical sectional outline is that the pressure gradient of warmer air in the interior of the tower is less than the pressure gradient of surrounding atmospheric air.
When the wind velocity at the top of the cooling tower increases to a medium or high value, the likelihood of penetration of cool air along the internal surface of the main wall is less pronounced. However, the rapidly flowing air currents are turbulent, and pronounced complex tubulence in the passage which is provided for escape of vapor-laden columns of ascending air reduces the effective cross-sectional area of the columns. This brings about the same drawbacks as if cool air were permitted to descend along the inner side of the main wall.